1. Field of the Invention
The present invention relates to a gas sensor and a gas concentration controller used to measure oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
In recent years, exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), as well as carbon monoxide (CO), carbon dioxide (CO.sub.2), water (H.sub.2 O), hydrocarbon (HC), hydrogen (H.sub.2), oxygen (O.sub.2) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO.sub.2.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO.sub.2 should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known as an instrument for detecting NOx. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive.
The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
The suggested conventional gas sensor is exemplified by a limiting current type oxygen sensor based on the use of an oxygen pump as shown in FIG. 17. The oxygen sensor comprises three stacked solid electrolyte layers 100a to 100c. The second solid electrolyte layer 100b is used as a spacer layer to possess a reference gas-introducing space 102 formed by side surfaces of the spacer layer 100b, an upper surface of the lowermost solid electrolyte layer 100a, and a lower surface of the uppermost solid electrolyte layer 100c. For example, the atmospheric air is introduced into the reference gas-introducing space 102 which is provided with an inner pumping electrode 104a formed on its inner wall surface. An outer pumping electrode 104b is formed on an upper surface of the uppermost solid electrolyte layer 100c. A diffusion rate-determining layer 106 is formed so that the electrode 104b is covered therewith. An oxygen pump 108 is constructed by the outer pumping electrode 104, the inner pumping electrode 104a, and the solid electrolyte layer 100c intervening therebetween.
Upon the operation of the oxygen sensor, a constant pumping voltage Vp is applied between the inner pumping electrode 104a and the outer pumping electrode 104b. A current flowing between the both electrodes 104a, 104b is measured by using an ammeter 110. Thus the oxygen concentration in exhaust gas is measured.
Upon the operation of the sensor described above, the constant pumping voltage Vp is applied. Therefore, for example, as shown in FIG. 18, when the oxygen concentration is increased, the amount corresponding to electromotive force is decreased by the amount corresponding to impedance of the oxygen pump 108. As a result, an oxygen concentration to be substantially controlled is increased. In such a situation, it is impossible to accurately measure the oxygen concentration (the oxygen concentration is higher at Point B than at Point A in FIG. 18).
On the other hand, Japanese Utility Model Publication No. 7-45004 discloses a system in which a voltage corresponding to a pumping current is generated by using an operational amplifier. The voltage is returned to the operational amplifier via a feedback resistor, and it is supplied to a resistor which is connected to a power source in series. When the pumping current is increased, the voltage generated by the resistor is superimposed and applied to the pump.
This system comprises a circuit as shown in FIG. 19. The output of the operational amplifier OP is returned to an input terminal on a side of an air electrode (inner pumping electrode 104a) via the feedback resistor R1 so that the voltage corresponding to the pumping current is generated at an output point A. On the other hand, the output is returned to an input terminal on a side of an outer pumping electrode 104b via the resistor R2, and the current is allowed to flow via the resistor r so that an amount of voltage generated in the resistor r is superimposed on a power source voltage V.sub.E.
When the resistor connected to the power source in series is appropriately designed, a voltage corresponding (actual pump impedance.times.pumping current) is superimposed on the pumping voltage Vp so that the operation point is set at any of certain flat portions on limiting current characteristic curves as shown in FIG. 20. Thus the oxygen concentration is measured with a high degree of accuracy.
However, in the case of the conventional gas sensor, when the oxygen concentration in a measurement gas is increased, the amount corresponding to voltage drop is increased, and it becomes far larger than the amount corresponding to electromotive force. Therefore, it is difficult to operate the gas sensor at an operation point which accurately corresponds to a certain electromotive force.
When the temperature of exhaust gas greatly changes as in the automobile, the gas sensor is provided with a heater, for which a mechanism for controlling the electric power to be supplied to the heater is provided, in some cases. Even when such a system is adopted, the impedance of the oxygen pump 108 is slightly changed. When the pumping current is increased, a large error occurs in correction for the amount corresponding to voltage drop. As a result, it is difficult to correctly measure the high oxygen concentration.
This problem is most serious especially when the oxygen pump 108 is used as an oxygen concentration controller. When the oxygen pump 108 is used as an oxygen pump, even if the oxygen concentration in the measurement gas is increased, the pumping current is increased, and the oxygen concentration in the measurement space is increased from 10.sup.-10 atm to 10.sup.-3 atm, then the change in current based on the change in oxygen concentration is about several % at most, as compared with the increased pumping current. However, when the oxygen pump 108 is used as an oxygen concentration controller, the change in oxygen concentration is exactly the large change from 10.sup.-10 atm to 10.sup.-3 atm as it is.
Practically, a problem arises in that it is impossible to superimpose the voltage corresponding to (pump impedance.times.pumping current) on the pumping voltage, and hence the accuracy is further decreased. FIG. 21 shows such a situation as analyzed in a comparative test. In this comparative test, the temperature of the gas sensor is adjusted so that the impedance of the oxygen pump 108 is 100.OMEGA. in any case.
In the conventional method (Japanese Utility Model Publication No. 7-45004), the correction voltage is ideally (100.OMEGA..times.pumping current) because the impedance of the oxygen pump 108 is 100.OMEGA.. However, in fact, correction is successful for only (50.OMEGA..times.pumping current) which is 1/2 of (100.OMEGA..times.pumping current).
Such unsuccessful correction is caused by oscillation. In a range of not less than (50.OMEGA..times.pumping current), the control system suffers an oscillation phenomenon, making it impossible to perform control.
In Japanese Utility Model Publication No. 7-45004, in order to measure the impedance of the oxygen pump, an amount corresponding to an alternating current (500 to 100 kHz) is superimposed on the power source so that the impedance of the oxygen pump is measured by using the alternating current voltage. However, oscillation tends to occur because the amount corresponding to the alternating current is subjected to positive feedback. For this reason, the output of the operational amplifier OP is subjected to positive feedback by the aid of a low pass filter so that the amount corresponding to the alternating current is eliminated. Thus only an amount corresponding to a direct current (for correcting voltage drop) is subjected to positive feedback, and an amount of voltage drop is superimposed on the pumping voltage Vp. In an experiment, the alternating current has a frequency of 10 kHz, and the low pass filter has a cut-off frequency of 1 kHz. In this system, the heater is not controlled on the basis of a signal of the amount corresponding to the alternating current.
According to the experiment, the oscillation phenomenon caused by the direct current component occurs at an extremely low frequency of not more than 50 Hz. Therefore, a problem of possible occurrence of oscillation due to the amount corresponding to the direct current still remains for the low pass filter which makes cutting for those having a frequency of not less than several hundreds Hz.
Further, this system requires an electric circuit comprising the low pass filter or the low pass filter+CR filter. It has been demanded to realize a simple system having a sufficient effect.
On the other hand, an all-range type oxygen sensor based on the use of an accurate oxygen pump is widely known as shown in FIG. 22. This oxygen sensor is formed with an internal space 124 for a pumping cell 120 and a sensor cell 122, and the internal space 124 communicates with an atmosphere of measurement gas via a diffusion rate-determining section 126.
Further, a sensor for measuring NOx is known, with which a gas (for example, NOx) including bound oxygen is measured by lowering the oxygen concentration in the gas to a constant low level by using an oxygen pump, and then further lowering the oxygen concentration to decompose NOx so that oxygen produced during the decomposition is measured by using an oxygen pump.
Such a sensor is provided with an oxygen concentration controller based on the use of the oxygen pump so that the oxygen concentration is controlled to be constant and low by using the oxygen concentration controller. Therefore, the oxygen concentration controller is required to have an accuracy which is equivalent to or higher than that of the oxygen sensor.
As for the all-range sensor, the pumping current is small in a range in which the oxygen concentration is low. Therefore, the accuracy is not lowered so much in such a range by the amount corresponding to voltage drop resulting from the impedance of the pump. On the other hand, in a range in which the oxygen concentration is high (for example several %), the accuracy is lowered due to the increased influence of the amount corresponding to voltage drop. However, no serious problem occurs even when an error is several hundreds ppm, because the oxygen concentration to be measured is several % (several ten thousands ppm).
On the contrary, for example, in the case of the NOx sensor which is used to measure the concentration of several thousands ppm at most, the change in oxygen concentration of the degree of several hundreds ppm brings about a large factor of error. Therefore, the oxygen concentration controller used for such a gas sensor is required to have a high degree of control accuracy.
As shown in FIG. 22, in the case of the oxygen sensor in which the oxygen concentration is controlled on the basis of an electromotive force generated between a measuring electrode 128 and a reference electrode 130, the pumping voltage (direct current voltage) Vp is subjected to feedback control so that a constant terminal voltage generated between the measuring electrode 128 and the reference electrode 130 is maintained. The oxygen sensor shown in FIG. 22 has a high degree of accuracy, however, it has a drawback that the control system suffers the oscillation phenomenon.
Namely, the feedback control is performed as follows. In general, a reference voltage as a target is compared by a comparator with the electromotive force generated between the measuring electrode 128 and the reference electrode 130. A difference obtained by the comparator is amplified to generate an amplified voltage on the basis of the difference from the target value. The amplified voltage is applied to the oxygen pump 132.
However, this system has a drawback that if the gain of the amplifier is set to be excessively large, the feedback control suffers oscillation.
This phenomenon is caused by the existence of any geometrical dimension of the measuring electrode 128 and the pumping electrode 134 contacting with the internal space 124. For example, when the oxygen concentration around the measuring electrode 128 is lower than the target value, the feedback control is performed so that the pumping voltage Vp is increased. Accordingly, the pumping voltage Vp is increased, the oxygen in the internal space 124 is pumped out, and the oxygen concentration in the internal space 124 is gradually decreased. However, the decrease in oxygen concentration is transmitted to a part of the space used for the measurement in a delayed manner due to the presence of the geometrical dimension described above. As a result, the oxygen concentration in the internal space 124 becomes lower than the target value. The lower oxygen concentration is detected by the measuring electrode 128 after a short delay period, and then the feedback control is performed so that the pumping voltage Vp is decreased.
In this case, the partial pressure of oxygen in the internal space 124 is gradually increased as well. However, a phenomenon occurs due to the geometrical dimension, in which the oxygen concentration in the internal space 124 has been excessively increased when the measuring electrode 128 detects the increase. As a result, the feedback control circuit suffers oscillation.